What is RISC and CISC Architecture

What is RISC and CISC Architecture

Central Processing Unit Architecture operates the capacity to work from “Instruction Set Architecture” to where it was designed. The architectural designs of CPU are RISC (Reduced instruction set computing) and CISC (Complex instruction set computing). CISC has the ability to execute addressing modes or multi-step operations within one instruction set. It  is the design of the CPU where one instruction performs many low-level operations. For example,  memory storage, an  arithmetic operation and loading from memory. RISC  is a CPU design strategy based on the insight that simplified instruction set gives higher performance when combined with a microprocessor architecture which has the ability to execute the instructions by using some microprocessor cycles per instruction.

What is RISC and CISC Architectures

Hardware designers invent numerous technologies & tools to implement the desired architecture in order to fulfill these needs. Hardware architecture may be implemented to be either hardware specific or software specific, but according to the application both are used in the required quantity. As far as the processor hardware is concerned, there are 2 types of concepts to implement the processor hardware architecture. First one is RISC and other is CISC.
CISC Architecture

The CISC approach attempts to minimize the number of instructions per program, sacrificing the number of cycles per instruction. Computers based on the CISC architecture are designed to decrease the memory cost. Because, the large programs need more storage, thus increasing the memory cost and large memory becomes more expensive. To solve these problems, the number of instructions per program can be reduced by embedding the number of operations in a single instruction, thereby making the instructions more complex.

• MUL loads two values from the memory into separate registers in CISC.
• CISC uses minimum possible instructions by implementing hardware and executes operations.
• Instruction Set Architecture is a medium to permit communication between the programmer and the hardware. Data execution part, copying of data, deleting or editing is the user commands used in the microprocessor and with this microprocessor the Instruction set architecture is operated.
• The main keywords used in the above Instruction Set Architecture are as below

Examples of CISC PROCESSORS

IBM 370/168 – It was introduced in the year 1970. CISC design is a 32 bit processor and four 64-bit floating point registers.

VAX 11/780 – CISC design is a 32-bit processor and it supports many numbers of addressing modes and machine instructions which is from Digital Equipment Corporation.

Intel 80486 – It was launched in the year 1989 and it is a CISC processor, which has instructions varying lengths from 1 to 11 and it will have 235 instructions.

CHARACTERISTICS OF CISC ARCHITECTURE

• Instruction-decoding logic will be Complex.
• One instruction is required to support multiple addressing modes.
• Less chip space is enough for general purpose registers for the instructions that are 0operated directly on memory.
• Various CISC designs are set up two special registers for the stack pointer, handling interrupts,  etc.
• MUL is referred to as a “complex instruction” and requires the programmer for storing functions.

RISC Architecture

RISC (Reduced Instruction Set Computer) is used in portable devices due to its power efficiency. For Example, Apple iPod and Nintendo DS. RISC is a type of microprocessor architecture that uses highly-optimized set of instructions. RISC does the opposite, reducing the cycles per instruction at the cost of the number of instructions per program Pipelining is one of the unique feature of RISC. It is performed by overlapping the execution of several instructions in a pipeline fashion. It has a high performance advantage over CISC.

RISC ARCHITECTURE CHARACTERISTICS

• Simple Instructions are used in RISC architecture.
• RISC helps and supports few simple data types and synthesize complex data types.
• RISC utilizes simple addressing modes and fixed length instructions for pipelining.
• RISC permits any register to use in any context.
• One Cycle Execution Time
• The amount of work that a computer can perform is reduced by separating “LOAD” and “STORE” instructions.
• RISC contains Large Number of Registers in order to prevent various number of interactions with memory.
• In RISC, Pipelining is easy as the execution of all instructions will be done in  a uniform interval of time i.e. one click.
• In RISC, more RAM is required to store assembly level instructions.
• Reduced instructions need a less number of transistors in RISC.
• RISC uses Harvard memory model means it is Harvard Architecture.
• A compiler is used to perform the conversion operation means to convert a high-level language statement into the code of its form.

The Advantages of RISC architecture

• RISC(Reduced instruction set computing)architecture has a set of instructions, so high-level language compilers can produce more efficient code
• It allows freedom of using the space on microprocessors because of its simplicity.
• Many RISC processors use the registers for passing arguments and holding the local variables.
• RISC functions use only a few parameters, and the RISC processors cannot use the call instructions, and therefore, use a fixed length instruction which is easy to pipeline.
• The speed of the operation can be maximized and the execution time can be minimized.
• Very less number of instructional formats, a few numbers of instructions and a few addressing modes are needed.

The Disadvantages of RISC architecture

• Mostly, the performance of the RISC processors depends on the programmer or compiler as the knowledge of the compiler plays a vital role while changing the CISC code to a RISC code
• While rearranging the CISC code to a RISC code, termed as a code expansion, will increase the size. And, the quality of this code expansion will again depend on the compiler, and also on the machine’s instruction set.
• The first level cache of the RISC processors is also a disadvantage of the RISC, in which these processors have large memory caches on the chip itself. For feeding the instructions, they require very fast memory systems.

Advantages of CISC architecture

• Microprogramming is easy assembly language to implement, and less expensive than hard wiring a control unit.
• The ease of microcoding new instructions allowed designers to make CISC machines upwardly compatible:
• As each instruction became more accomplished, fewer instructions could be used to implement a given task.

Disadvantages of CISC architecture

• The performance of the machine slows down due to the amount of clock time taken by different instructions will be dissimilar
• Only 20% of the existing instructions is used in a typical programming event, even though there are various specialized instructions in reality which are not even used frequently.
• The conditional codes are set by the CISC instructions as a side effect of each instruction which takes time for this setting – and, as the subsequent instruction changes the condition code bits – so, the compiler has to examine the condition code bits before this happens.

Comparison of RISC and CISC Architecture